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2005 kx250f weird 1/2 throttle hiccup

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So i just rebuilt my top end on my 2005 kx250f. SS valves, pro-x stock piston and rings replacement, and an FMF powerbomb header and pipe other than that bike is stock. i pulled the carb apart and it had a pro-circuit jetting set-up so i went through and replaced all the jets back to stock except the pilot jet.

Main jet 178

pilot is a 42 from a 40

OBEKS needle from clip position from top

leak Jet is a 60

i also put in a moose racing adjustable air/fuel screw that is out 2 1/4 turns.

when i am riding if i am at half throttle cruising at a single rpm the bike hiccups at a single rpm. Then when i snap the throttle open from half throttle it hesitates a second, hiccups then snaps open like a 2 stroke hittting a powerband. i thought i may have fouled a plug. so i pulled the old plug out and inspected it and it looked like it had been burning just fine. not rich or lean. so i put a new plug in it and the same thing happened. William1 i would like your opinion on this please. anyone else wanna chime in on what i should be looking for and tuning this thing out i'm all ears. Thanks.

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Response from Krannie on a carb issue I had, should have good information in here for your problem as well
Keep in mind I have a 06 KXF450, so some of his recommendations don't apply, but some do.

Fuel screw needs to be within 1-2.5 turns out, no more. If you need more, your pilot jet is clogged or too small.

Follow the fuel screw adjustment procedure:

http://www.thumpertalk.com/forum/sho...d.php?t=327405

If you get bog rolling on the throttle, it's fuel starvation to the float bowl.

If you get bog when the throttle is 'whacked' under load, the issue is fuel starvation of the Accelerator pump, or a clogged pump nozzle (or broken linkage parts, which is unlikely).

Tilt up the subframe so you can see inside the carb. With the motor off, gas on, using a flashlight, look to see if a stream of gas shoots down the carb throat when you whack the throttle. It should do it every time, no matter how many times or how fast you whack it. It should not hit the slide, ever.

It should appear to shoot 6 ft into the motor.

http://www.youtube.com/watch?v=mSZAfjRcWFU

If it dribbles out of the nozzle, it's most likely clogged at the nozzle, or the apump diaphragm is torn. Remove apump cover and inspect.

You should have a 50 or smaller leakjet to get good results.

Bigger makes it too short of a squirt, smaller makes it too long.

The apump linkage can be fine tuned for better results, but should be close enough from the factory to be ride-able, unless someone has been playing with the carb.

If it is clogged, you need to remove the carb and clean with compressed air and brake cleaner.

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themitch seems to know what he is talking about;) Though I'll add, fuel screw is for idle only. As you get off of idle, its' minimal effect diminshes to the point where at 1/4 thottle, the pilot could be plugged, the fuel screw closed and you'd never know it.

Leak jet size (going smaller) not only increases duration but increases volume and sensitivity as well.

Just to clarify, your bike runs flawlessly at steady throttle, only when you goose the gas, does it balk, right?

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I will run it up to about half throttle and then snap it open then it will hiccup. i dont really notice anything down on the bottom end. it feels like a 2 stroke right before it hits the powerband. real sluggish then snaps real hard WOT in a instant. it should feel like constant power all the way through. dont really notice it on bottom end where the pilot is used.

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Ok, for starters, the pilot is for idle only. Once you get off idle, you are going on the needle. Up to 1/4 throttle, it is the straight diameter (to vary this area, you need to change needles), after thant it is the taper and you can vary this section some by moving the clip.

Now, are you positive it is 1/2 throttle or is that a guess? To tell for sure, you put tape on the grip and throttle housing and mark it off in 1/8ths of total travel.

You need to be positive about where this is happening or we will be chasing our tails.

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I will hop on the bike today and take it for a ride and will for sure know and let you know tomorrow.

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LOL, mark the grip first. After the tape is on, I put a single line on the housing top. Then a line on the grip, marked with a 0, then I twist the throttle WOT. I mark a line, and place an 8 there. I divide the distance in half, that becomes the 4, then the remaining space in quarters, above half throttle for 5, 6, 7 and below for 1, 2, 3.

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Well i took the bike out this weekend and basically every position of the throttle bogs until i snap it wide open. felt like a 2 stroke coming off the bottom end and snaps wide open when it hits the powerband. it bogs every gear no matter what. gotta slip the clutch a lot to get it past the bog. gonna pull the pilot out tonight and see if its clogged. hope thats the case cause then it would be an easy fix.

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Ok

First, check/reset the AP timing to 'stock':

Resetting the AP timing to 'stock'

Method #1 - Most accurate

Turn the idle speed screw so the slide is completely closed (count the turns and note it) .

Turn the throttle wheel, raise the slide, slip a .8mm drill blank under the slide, release the throttle wheel, pinning the blank.

Turn the AP timing screw so the play just barely goes away.

Remove the drill blank, reset the idle speed to where it was.

Fine adjust the AP timing screw by test riding, turning in 1/2 turn increments to where the bog is just gone. Should be less than 2 turns, if at all.

Method #2 Not as accurate

Back out the idle speed knob (count the turns and note it) until the screw end of the idle speed cable is no longer touching the theottle wheel - Throttle cables may have to be fully lossened.

Ensure the slide has bottomed out on the carb bore'

Set the timing gap between the cam and the screw to be .010"

Reset the idle speed per your notes, reset the throttle cables if loosened.

Next, either do the oring mod, wire the linkage or best, put in a stiffer AP cam spring.

No test ride the bike, make sure it is fully heated. Ride normally. If the bog is still there, turn the AP timing screw 1/2 turn CCW and retest. If it is worse, turn it 1 full turn CW. If it was better but still bogs, turn it another 1/2 turn CCW. The 'stock' AP timing adjustment is very close. Now you have to get it just right by tweaking and testing.

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So do we think it is the AP timing? where do i get a stiffer spring at? i wont have anytime til this weekend. do i need to adjust with the bike on and running? or just do the initial setup and fine tune after running it?

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A #60 leak jet is /should be fine.

Stiffer springs are available from Merge Racing. In the interm, the Oring mod works fine. Hit Lowes or a plumbing store for some #78 orings.

Check the current AP timing, adjust if needed. This is done in your garage.

Now hit your riding area!

Get the bike hot by a fifteen minute ride. Check for the bog. If it is there, turn the timing screw ccw 1/2 turn. If it is worse, return it to the base setting and go CW 1/2 turn. Bottom line is, which ever directio improves it, go that way. Make a note of which direction and how much you turn it. Test ride NORMALLY (do not try to force a bog) after each incremental adjustment.

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OK i checked the AP timing this weekend and it doesnt seem to be off at all really. its not hitting the slide or the needle. reset my fuel screw to 2.5 turns out. went out for a ride and still a bog. i dont know how to explain the bog other than how it would feel in a car. it feels like what a fat O2 (oxygen) sensor would feel on a car. got back from the ride and immediately pulled out my spark plug and it seems to be burning perfect. tinted dark brown on top. not white like a lean condition and not black and sooty like a rich condition. but just right. although what concerned me was there was this white film around the spark plug on the hex before the threads. ???? this is starting to irritate me. can you help me out william1 or anybody else?

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it seems like when i am at a consistent rpm and i roll on it, not even snap it open, i get a bog then a hesitation and as i roll harder on the throttle the RPMS clear it up and it snaps open like it is running too fat. at a consistent RPM it cuts out -----:ride:----:lol:---. this looks ridiculous but i have no way of recording what i hear while i'm riding. lol

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No where did I say to look to see if the squirt was hitting the slide.

Did you do the oring or similar?

Is you base jetting correct, ie, the bike runs faultlessly under load, at steady throttle?

Was your fuel screw set according to the proper procedure and not just 'set' to some arbitrairy setting? You have the correct sized pilot in?

I explained how to reset/check the AP. You have to follow the instructions. You cannot wing it. Fional adjustment s done when riding. Not in the garage.

You cannot read a plug with pump fuel. It always will look perfect. The white you saw was a little corrosion. Ignore it.

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ok william1 i have looked up the AP timing procedure in my factory service manual and it calls for "Select an arbor [A] of the same diameter as the throttle

valve height and insert it under the throttle valve.

Throttle Valve Height – 1.25 mm (0.0492 in.)" so is this not correct to set it to this height and then take the play out of the adjustment screw? also i got some #5 o-ringsd yesterday from the hardware store which i am going to install once i have the the AP timing adjusted.

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These are the two methods

Resetting the AP timing to 'stock'

Method #1 - Most accurate

Turn the idle speed screw so the slide is completely closed (count the turns and note it) .

Turn the throttle wheel, raise the slide, slip a .8mm drill blank under the slide, release the throttle wheel, pinning the blank.

Turn the AP timing screw so the play just barely goes away.

Remove the drill blank, reset the idle speed to where it was.

Fine adjust the AP timing screw by test riding, turning in 1/2 turn increments to where the bog is just gone. Should be less than 2 turns, if at all.

Method #2 Not as accurate

Back out the idle speed knob (count the turns and note it) until the screw end of the idle speed cable is no longer touching the theottle wheel - Throttle cables may have to be fully lossened.

Ensure the slide has bttomed out on the carb bore'

Set the timing gap between the cam and the screw to be .010"

Reset the idle speed per your notes, reset the throttle cables if loosened.

Your manual is referring to method one. Using a drill blank that large will make a squirt very late. Try either one of the ones I presented.

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